1. Field of the Invention
This invention relates to radio frequency (RF) radiation sources, and more particularly to slow wave tubes that are coupled with electron beams to provide an RF source and to related energy extraction mechanisms.
2. Description of the Related Art
High power slow wave structures commonly consist of a cylindrical waveguide with a periodically varying inner wall radius in the form of a periodic series of ripples. These structures support electromagnetic waveguide modes, with some waves having phase velocities less than the speed of light. Slow wave tubes have been coupled with electron (e.sup.-) beams directed along the tube axis to generate RF power, specifically in the microwave regime (10.sup.9 -10.sup.11 Hz).
Two devices of this type are generally categorized as backward wave oscillators (BWOs) and travelling wave tube amplifiers (TWTAs), and are described in J. Swegle et al., Phys. Fluids, 28(9), Sep. 1985, pages 2882-2894, and W. W. Destler et al., "Microwave and Particle Beam Sources and Propagation" SPIE Vol 873, (1988), pages 84-91. With a BWO, a spontaneous generation of microwave power occurs at a frequency that is determined by a combination of the tube geometry and the e.sup.- beam current and voltage. In plasma-filled devices the frequency is also dependent on the plasma density. Instabilities occur within the tube when the e-beam's slow space charge wave has the same phase velocity as a structure mode. Under these conditions the beam's slow space charge wave can develop, resulting in the deceleration of beam electrons as beam bunching occurs. The decelerated electrons release energy which is systematically coupled into the electromagnetic wave field of the slow wave structure. These fields, which have an axial electric field component, enhance the bunching of the beam's space charge and thereby further decelerate the beam electrons, thus transferring more energy into the wave fields. As the positive reinforcement cycle continues, the structure's electromagnetic fields exponentially increase in amplitude at the frequency of the beam-structure resonance, resulting in a spontaneous generation of microwave power.
In a BWO, the coupling of the slow space charge wave with the tube's modes occurs when the structure mode has a negative group velocity. This results in a transfer of e.sup.- beam energy to the electromagnetic wave field in a direction that is backward, or opposite, to the direction of the e.sup.- beam. The spontaneous generation of microwave power grows out of e.sup.- beam noise and the structure's internal feedback, with no input RF signal. The backward traveling RF wave is reflected off either the e.sup.- beam generator itself, or off a smaller diameter section of the tube near the e.sup.- beam generator which acts as a waveguide below cutoff for the operating frequency of the device as shown in FIG. 1. It then travels in a second pass through the tube for extraction at the opposite end of the tube from the e.sup.- beam generator.
If the e.sup.- beam's slow space charge wave couples with a tube mode that has a positive group velocity, as opposed to the negative group velocity associated with a BWO, the slow wave structure is commonly known as a TWTA. In this case the transfer of e.sup.- beam energy to the wave field is forward, or co-directional, with the direction of the e.sup.- beam. RF excitation from an external source must be launched into the tube near the end that receives the e.sup.- beam for the signal to be grown exponentially as it propagates (in a single pass) along the length of the structure.
A simplified sectional view of a conventional structure that can function as a BWO is given in FIG. 1. An electron gun 100 transmits an e.sup.- beam through an internally rippled slow wave tube 102. Functioning as a BWO, a backward traveling RF wave is reflected off the end of the (smaller diameter) unrippled tube section 104 adjacent the e.sup.- gun, and emerges from the flared tube outlet 106. A structure that can function in a TWTA mode is shown in FIG. 2, in which the same reference numerals are used to indicate the same elements as in FIG. 1. An input RF signal is coupled into the side of the tube through a coupling port or ports 108 into a region which can propagate the RF signal downstream into the slow wave structure. To restrict the signal from propagating upstream and entering the e.sup.- gun, the smaller diameter tube section 104 which cannot propagate the RF signal is positioned upstream of the coupling region and downstream of the e.sup.- gun. In this mode the RF signal grows as it propagates forward through the tube, and is emitted without a reflection of its propagation direction.
There are several undesirable limitations of both the BWO and TWTA. Since the RF signal is extracted from the same end of the slow wave tube where the e.sup.- beam is collected, special provisions must be made to extract the RF signal while terminating the e.sup.- beam. In magnetized devices the beam can be propagated out of the confining magnetic field region and allowed to expand, striking the waveguide wall. In alternative approaches various types of "beam dumps" in the form of plugs at the end of the waveguide tube have been employed, but they are difficult to design so that they do not interfere with the RF signal. In both elimination methods the e.sup.- beam impacting the waveguide or "dump" surface tends to result in the formation of plasma (which can adversely effect the radiation of RF power) at the end of the tube by partially reflecting, absorbing and/or scattering the RF signal.
The permissible average e.sup.- beam power and energy, and thus the amount of average power that can be transferred to the RF signal, is also limited by the need to collect the e.sup.- beam at the same location at which the RF signal is extracted. The use of a simple water cooling system for the beam collector would permit operation at a higher e.sup.- beam duty cycle and average power. However, water cooling systems which require metal structures for effective heat transfer or re-circulating liquid coolant must be positioned in the throat of the RF radiating aperture and thus can interfere with the extraction of the RF signal. This limits the use of water cooling, with a consequent reduction in the duty cycle and average powers that might otherwise be achieved.
In the case of a BWO, the backward direction of the RF signal as it is originally generated also leads to inefficiencies. The need to reflect the backward propagating RF wave at the input end of the tube, and then allow it to travel in a second pass to the outlet end of the tube, can result in a reduction in signal amplitude through wall losses, as well as adversely affecting the structure's conversion efficiency.
The RF signal extracted from the slow wave tube is fundamentally in the TM.sub.01 (cylindrical waveguide) mode established by the dynamics of the RF generating interaction between the electron beam's space charge wave and the cylindrical waveguide's electromagnetic field components. However, a rectangular TE.sub.10 mode is generally preferred for radiating RF signals, since this mode can be easily managed and fed directly to antenna feeds for radiation. A separate mode converter is thus necessary to place the generated RF signal in the desired mode format for radiation.
A BWO with TE.sub.10 extraction waveguides at the opposite end of the tube from the e-beam source is illustrated in Phelps, "More Than 10 GW from a Relativistic BWO?" Third National Conference on High Power Microwaves Digest, December 1986, pages 240-244 (FIG. 6). However, the extraction waveguides require the radiation to undergo two full passes through the BWO before it can be extracted, additional unwanted modes could be excited, and only individual extractions are disclosed. An article in the same publication by Voss et al., "Characterization of a High Power Microwave Cross-Field Oscillator Operated at S-Band" pages 147-148, shows a slow wave tube with a single lateral extraction structure. With this device there would be an excitation of unwanted modes over appreciable bandwidths, there is no provision for multiple phase coherent outputs that can be used for a phased array antenna, the amount of power that can be extracted is quite limited, and the system is asymmetrical. In Wharton and Butler, "Relativistic O-Type Oscillator-Amplifier Systems" Intense Microwave and Particle Beams, Vol. 1226, Bellingham, Wash., 1990, page 23, two TWT amplifiers are driven by a single relativistic BWO master oscillator. Microwave samples are obtained from extraction ports near the input end of a BWO and coupled into respective TWTAs. There is no disclosure of any TM.sub.01 -to-TE.sub.10 mode conversion, the extraction will excite propagations other than (TM.sub.01 cylindrical waveguide modes) over appreciable bandwidths, the two output samples are used independent of each other, and again the outputs are not compatible as inputs to a phased array antenna.